/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> ans;
        if(root==nullptr)
            return ans;
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty())
        {
            int len=q.size();
            vector<int> stk;
            for(int i=0;i<len;i++)
            {
                TreeNode* node=q.front();
                q.pop();
                if(node->left)
                    q.push(node->left);
                if(node->right)
                    q.push(node->right);
                stk.push_back(node->val);
            }
            ans.push_back(stk);
        }
        for(int i=0;i<ans.size();i++)
        {
            if(i&1)
            {
                reverse(ans[i].begin(),ans[i].end());
            }
        }
        return ans;
    }
};